os/exec: start checking for context cancelation in Start

Previously we started checking for context cancelation in Wait, but
that meant that when using StdoutPipe context cancelation never took
effect.

Fixes #16222.

Change-Id: I89cd26d3499a6080bf1a07718ce38d825561899e
Reviewed-on: https://go-review.googlesource.com/24650Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/exec/exec.go

@@ -111,6 +111,7 @@ type Cmd struct {
 	closeAfterWait  []io.Closer
 	goroutine       []func() error
 	errch           chan error // one send per goroutine
+	waitDone        chan struct{}
 }
 
 // Command returns the Cmd struct to execute the named program with
@@ -326,6 +327,15 @@ func (c *Cmd) Start() error {
 	if c.Process != nil {
 		return errors.New("exec: already started")
 	}
+	if c.ctx != nil {
+		select {
+		case <-c.ctx.Done():
+			c.closeDescriptors(c.closeAfterStart)
+			c.closeDescriptors(c.closeAfterWait)
+			return c.ctx.Err()
+		default:
+		}
+	}
 
 	type F func(*Cmd) (*os.File, error)
 	for _, setupFd := range []F{(*Cmd).stdin, (*Cmd).stdout, (*Cmd).stderr} {
@@ -361,6 +371,17 @@ func (c *Cmd) Start() error {
 		}(fn)
 	}
 
+	if c.ctx != nil {
+		c.waitDone = make(chan struct{})
+		go func() {
+			select {
+			case <-c.ctx.Done():
+				c.Process.Kill()
+			case <-c.waitDone:
+			}
+		}()
+	}
+
 	return nil
 }
 
@@ -410,20 +431,9 @@ func (c *Cmd) Wait() error {
 	}
 	c.finished = true
 
-	var waitDone chan struct{}
-	if c.ctx != nil {
-		waitDone = make(chan struct{})
-		go func() {
-			select {
-			case <-c.ctx.Done():
-				c.Process.Kill()
-			case <-waitDone:
-			}
-		}()
-	}
 	state, err := c.Process.Wait()
-	if waitDone != nil {
-		close(waitDone)
+	if c.waitDone != nil {
+		close(c.waitDone)
 	}
 	c.ProcessState = state
 

src/os/exec/exec_test.go

@@ -878,3 +878,73 @@ func TestContext(t *testing.T) {
 		t.Fatal("timeout waiting for child process death")
 	}
 }
+
+func TestContextCancel(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	c := helperCommandContext(t, ctx, "cat")
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	c.Stdin = r
+
+	stdout, err := c.StdoutPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	readDone := make(chan struct{})
+	go func() {
+		defer close(readDone)
+		var a [1024]byte
+		for {
+			n, err := stdout.Read(a[:])
+			if err != nil {
+				if err != io.EOF {
+					t.Errorf("unexpected read error: %v", err)
+				}
+				return
+			}
+			t.Logf("%s", a[:n])
+		}
+	}()
+
+	if err := c.Start(); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := r.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := io.WriteString(w, "echo"); err != nil {
+		t.Fatal(err)
+	}
+
+	cancel()
+
+	// Calling cancel should have killed the process, so writes
+	// should now fail.  Give the process a little while to die.
+	start := time.Now()
+	for {
+		if _, err := io.WriteString(w, "echo"); err != nil {
+			break
+		}
+		if time.Since(start) > time.Second {
+			t.Fatal("cancelling context did not stop program")
+		}
+		time.Sleep(time.Millisecond)
+	}
+
+	if err := w.Close(); err != nil {
+		t.Error("error closing write end of pipe: %v", err)
+	}
+	<-readDone
+
+	if err := c.Wait(); err == nil {
+		t.Error("program unexpectedly exited successfully")
+	} else {
+		t.Logf("exit status: %v", err)
+	}
+}